This paper presents deposition of a-Si:H and nano-crystalline Si thin films for low cost and high efficiency solar cells in the second generation and third generation. For the deposition, we have developed a nanoparticle-controlled-plasma CVD reactor, by which we can control contribution of nanoparticles, formed in discharges, to films deposited [1]. In the upstream region from the discharges, a-Si:H films are deposited without incorporating a-Si:H nanoparticles into them (referred to as nanoparticle-free a-Si:H films) using SiH4(20-100%)+H2 gases. In the downstream region, nano-crystalline Si thin films are deposited with incorporating nano-crystalline Si nanoparticles into them using SiH4(< 5%)+H2 gases.
A-Si:H solar cells in the second generation have three important problems to be solved: 1) a low stabilized efficiency below 10%. 2) light-induced decrease in efficiency around -20% of the initial one. 3) a low deposition rate of 0.5 nm/s. Our nanoparticle-free a-Si:H films show remarkably high stability, that is, the films have an initial defect density of 5x1015 cm-3 and show no light-induced-increase in the defect density. Therefore, we have succeeded in overcoming the second problem mentioned above.
We have deposited "nano-crystalline silicon films", which are crystalline silicon nanoparticles embedded into a-Si:H for solar cells in the third generation. Nanoparticles are produced in hollow discharges, then they are transported to the substrate by gas flow, and eventually co-deposited there with SiH3 radicals. In this experiment, their mean size and crystallinity are 2 nm and 100%, respectively. A volume fraction of nanoparticles incorporated into the films is controlled by the discharge power or by a distance between the discharge region and the substrate. We have realized a volume fraction range of 0.03% to 60%. The films have an initial defect density of 3x1015 cm-3 and show high stability.
In conclusion, plasma control for nanoparticle formation is the key to deposition of such two kinds of Si films.
[1] M. Shiratani, et al., Thin Solid Films 427, 1 (2003).
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